The formation of scale in phosphoric acid plants and plants handling phosphoric acid has been a major problem in the phosphoric acid industry for decades. The reaction of phosphate ores such as a fluorapatite Ca10(PO4)6(F,OH)2 and/or a francolite Ca10(PO4)6-x(CO3)x(F,OH)2+x with sulfuric acid produces the desired phosphoric acid along with suspended and/or dissolved solids. Phosphoric acid concentration is normally expressed as percent by weight of P2O5.
The basic equation for the reaction for production of phosphoric acid from sulfuric acid and calcium ions and phosphate ions from ores can be written:3Ca2++2PO43−+6H++3SO42−→2H3PO4+3CaSO4 
The reaction between sulfuric acid and phosphate ore is normally carried out in an attack tank at elevated temperatures. After reaction, the phosphoric acid normally contains dissolved and suspended solids—the majority of the latter are filtered off. The filtered acid is then passed to an evaporator, where it is concentrated. The acid is heated by passing it through a heat exchanger heated by pressurized steam; the phosphoric acid is subsequently concentrated under reduced pressure in a vapor or flash chamber in the evaporator, where water evaporates from the phosphoric acid so that the acid is concentrated. Further suspended solids and/or dissolved solids in the phosphoric acid precipitate during the concentration process and deposit as scale on the surface of the heat exchanger and/or chamber surfaces. Typically such scale comprises polymorphs of calcium sulfate and several other minerals. Over time, the scale that accumulates on the surface of the heat exchanger reduces the efficiency thereof. Production needs to be stopped and the evaporator shut down for cleaning, an interruption that results in lost production time and increased production costs. Further, the scale that forms is extremely hard and adheres tenaciously to the surfaces on which it has deposited, and is thus intractable and difficult to clean from the surfaces. Further, the scale that deposits and accumulates is of relatively low water-solubility. Cleaning thus involves a considerable mechanical effort (hydroblasting), resources such as energy, water and materials, and manpower. One type of cleaning involves boil-out with dilute sulfuric acid. Furthermore, hard scale accumulated on heat exchanger surfaces is implicated in the breakage of fragile impregnated graphite tubes, which the heat exchanger may comprise.
Chemicals such as scale-inhibitors can be added to the phosphoric acid to mitigate scale build-up, but do not completely prevent the formation of scale that deposits on various surfaces. Therefore there is a need for new processes and materials that prevent, reduce, and/or slow formation and/or accumulation of scale on surfaces in contact with phosphoric acid in phosphoric acid plants or phosphoric acid sourced therefrom. There is a need to ameliorate the effects of scale accumulated on and adhering to heat exchanger and other surfaces of evaporators, pipes and the like that are or have been in contact with phosphoric acid made from the reaction of phosphate ores and sulfuric acid.